1. Field of the Invention
This invention relates to moulding processes and apparatus; and to moulded products produced therefrom. More particularly, this invention relates to moulding processes, especially injection moulding processes, and apparatus for producing oriented, moulded products, especially such products which have substantial volume but low surface area: volume ratio.
In this application, the following conventional abbreviations are used:
HDPE: high density polyethylene PA0 TPX: polytrimethylpentene PA0 PEEK: poly(aryletherketone) PA0 PVF: polyvinylfluoride PA0 PVdF: polyvinylidenedifluoride PA0 LDPE: low density polyethylene PA0 UHMWHDPE: ultrahigh molecular weight high density polyethylene PA0 PTFE: polytetrafluoroethylene.
2. Description of the Related Art Including Information Disclosed Under 37 C.F.R. .sctn..sctn.1.97-1.99.
It is well known that the properties, notably mechanical properties such as tensile modulus and strength, of a thermoplastic material, especially a semicrystalline polymeric thermoplastic material, may be enhanced in a given direction by causing the material to be oriented in that direction. Many processes have now been devised for accomplishing this enhancement of mechanical properties either by forming the material in the mass ab initio in an oriented state or by subsequently imparting plastic strain to the solid material. All such processes provide, or seek to provide, oriented product of comparatively simple, and constant, cross-section: examples are fibre and film, including biaxially oriented film; and rod, tube and sheet stock. No comparable benefit has hitherto been available for thermoplastic materials moulded from the melt.
It is also well known that successfully moulding a thermoplastic, especially a semicrystalline and/or filled polymeric thermoplastic, material from the melt to a cavity of substantial volume but low surface area: volume ratio is fraught with difficulty because the cooling in the mass of material of low thermal conductivity is not easy to control and because, as a result, contraction occurs in the mass as solidification proceeds (which contraction can be exacerbated by crystallite formation). This can result in sinking of the mould surface and both macroporosity and microporosity throughout the moulded product.
Moreover in relation to conventional injection moulding processes, the molten mass of mouldable material is injected into the mould cavity from one feeding point and the subsequent packing force is also applied at this single point. For certain requirements of mould design, in particular moulds with long flow paths and moulds with variation in cavity wall thicknesses the single feed may be split so that the cavity can be filled satisfactorily from a number of feeds, or gating points. This practice results in the production of internal weld lines within the moulded part, at the positions where the various melt flow fronts from the multiple gate points meet. It has been shown that the presence of weld lines can cause undesirable discontinuities in the mechanical properties of the moulded article.